1. Field of the Invention
The present invention relates to technique for substantially improving gradation reproducibility of a halftone image by partially exposing each elementary area which serves as a unit for a value of screen pattern data.
2. Description of the Prior Art
In an apparatus for recording a halftone image such as a graphic arts scanner of the electronic control type, halftone dots are produced by exposing a photosensitive material while turning an optical exposure beam on and off in response to the result of comparison between an image signal to a prescribed screen pattern signal. The screen pattern signal is generated on the basis of screen pattern data prepared in advance.
As is well known in the art, the screen pattern data express threshold values for respective small areas A.sub.p within a halftone dot HD illustrated in FIG. 1. The small areas A.sub.p (hereinafter referred to as "elementary areas") are formed by dividing the halftone dot HD into a matrix of elementary areas. FIG. 2 illustrates screen pattern data prepared for the halftone dot HD and stored in a memory. Numerals in this figure represent data values P assigned to respective elementary areas A.sub.p. Referring to FIG. 1, shaded areas have respective screen pattern data P which are at most 12, whereby the shaded areas are exposed with an optical exposure beam when an image signal having a value of 12 is supplied for the halftone dot HD.
The size of the elementary area A.sub.p is determined according to a spot diameter d of the optical exposure beam focused on the photosensitive material. In more concrete terms, the size of the elementary areas A.sub.p is so determined that a value of the spot diameter d is in the range from about "a" (hereinafter referred to as "elementary area size") of one side of each elementary area A.sub.p to about twice of "a". It is to be noted that the spot diameter d is assumed to be equal to the diameter of a circle inscribed in the elementary area A.sub.p in the drawing, for convenience of illustration.
Adjudgement is made as to whether the optical exposure beam is turned on or off for every scanning advance corresponding to the spot diameter d, whereby the intensity of the optical beam is changed in response to the result of the judgement. In the conventional method of recording a halftone image, therefore, a spatial interval which is a unit of the intensity change of the optical beam is limited to integral multiples of the elementary area size a.
Thus, when the relation (1) specified below holds for a square halftone dot having a screen pitch K, a gradation number M defined by the equation (2) (below) is the upper limit of number of levels for gradation expression: EQU K=na (1) EQU M=(K/a).sup.2 =n.sup.2 ( 2)
where n is an integer.
In order to increase the gradation number M, therefore, it is necessary to increase (K/a) in the equation (2). In a scanner of a flat bed type, however, the focal length of an image forming lens in a recording optical system is rather long in order to obtain a desired scanning length. Thus, the spot diameter d of the optical exposure beam cannot be reduced to a desired degree in the flat-bed type scanner. For example, providing a spot diameter of about ten to twenty micrometers poses significant technical difficulties and an inordinate increase in cost. Consequently, it is unavoidable in the flat-bed type scanner that the elementary area size a is relatively increased and the gradation number M is reduced, as is evident from the equation (2).
Also, in a drum type scanner which can employ relatively small valved spot diameter d (and the elementary area size a as the result), the gradation number M is small if the screen pitch K is small.
Thus, in the conventional apparatus for recording a halftone image, the gradation number M remains small when the ratio of the screen pitch K to the elementary area size a cannot be substantially increased, whereby gradation reproducibility of the image remains rather poor.
Even if the spot diameter d of the optical exposure beam can be reduced, the amount of the screen pattern data must be increased in order to increase the gradation number M, and hence a capacity of a screen pattern memory must be increased, which causes a considerable increase in cost.